Conventional rectifiers have relatively low power factors which limit the power that can be drawn from an AC line to a fraction of the rating of the line. Furthermore, highly distorted AC line currents are drawn by these conventional rectifiers, often causing interference with other electrical equipment in addition to equipment overcurrents and overvoltages. Techniques for improving power factor include passive waveform shaping methods, i.e., using input filters, and active methods, i.e., using boost or buck converter topologies. Such conventional active methods of obtaining high power factor generally employ a completely separate up-front converter to attain the high power factor followed by a DC-to-DC converter to produce the desired regulated DC output voltage. Thus, the power is converted twice, which is costly and inefficient. Moreover, the up-front converter must convert the entire delivered power. In fact, it must convert a peak power equal to twice the average power delivered.
A power conversion system employing a single power stage while operating at high power factor is described in commonly assigned U.S. Pat. No. 4,642,745 issued on Feb. 10, 1987 to R. L. Steigerwald and W. P. Kornrumpf. The power conversion system of the Steigerwald and Kornrumpf patent includes: a full-wave AC rectifier; a DC-to-AC converter; a transformer having a primary winding, a closely-coupled secondary output winding and a loosely-coupled secondary boost winding and a resonant capacitor coupled to the secondary boost winding. The secondary output winding is controlled by pulse width modulation (PWM), and the secondary boost winding is controlled by frequency modulation, thus allowing relatively independent control of the input current and output voltage.
Another form of dual output power converter is disclosed in U.S. Pat. No. 5,113,337 to Steigerwald which completely decouples the boosting converter and the power output stage. In this latter patent, the converter has a high power factor drawing high quality current waveforms from the AC source while providing a regulated DC output with fast transient response. One form of control for the power converters of the above cited patents is disclosed in each of the patents. In general, the patents describe a PWM control for controlling the regulated DC output voltage by adjusting the duty cycle of the inverter output signal whenever an error is detected between the commanded DC output voltage Vo* and the actual DC output voltage Vo. In addition, the Steigerwald and Kornrumpf patent describes an active frequency control wherein the actual converter input current is compared with a commanded current in phase with the utility line voltage. Any difference causes a frequency adjustment for controlling a boosting converter. Advantageously, since the boosting converter and the power supply output voltage are completely decoupled, any adjustment of the DC output voltage using the Steigerwald and Kornrumpf control system will have no effect on the input current to the converter.
Although the above described control scheme effectively controls a dual output converter to provide output voltage regulation at high power factor, it is desirable to provide a more simple control system which decouples the ac line current and the output dc voltage, while maintaining, or improving, output regulation and power factor.